1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system in which a main signal and an auxiliary signal such as a supervisory information signal are transmitted together from an optical transmitter to an optical receiver, with or without the intervention of an optical repeater, each being equipped with an optical amplifier that directly amplifies the optical signal.
2. Description of the Related Art
In the transmission of an optical signal, an auxiliary signal such as a supervisory information signal is transmitted, together with the main signal, from an optical transmitter (hereinafter referred to as the transmitting terminal) and the auxiliary signal thus transmitted is detected by an optical receiver (hereinafter referred to as the receiving terminal). When the transmitting and receiving terminals are separated by a long distance, an optical repeater installed between them extracts the auxiliary signal, and optically amplifies the main signal using an optical amplifier in the optical repeater. The amplified main signal is then transmitted, by the optical repeater, together with its response information corresponding to the supervisory information signal as an auxiliary signal. The reliability of the whole system is thus enhanced.
The following methods have been proposed in the prior art for the transmission of a supervisory information signal from an optical transmitter to an optical receiver, with or without the intervention of an optical repeater, each incorporating an optical amplifier.
i) A method in which the supervisory information signal is superimposed on the main signal, thereby transmitting the main signal together with the supervisory information signal using light of a single wavelength.
ii) A method in which supervisory information signal light of a wavelength different from that of the main signal light is wavelength-division multiplexed with the main signal light using a wavelength-division multiplexer, and at the receiving terminal, the signals are demultiplexed by a wavelength-division demultiplexer to detect the supervisory information signal/response signal.
An optical amplifier is constructed using an optical fiber doped with a rare earth element such as erbium, and is provided at its input side with an optical branching device for monitoring the condition of an input optical signal and at its output side with an optical band-pass filter for blocking the spontaneous emission light occurring within the optical fiber.
When amplifying input light in the transmitting terminal, receiving terminal, and/or optical repeater, the optical amplifier, because of its properties, amplifies not only the main signal component but also the signal components on both sides of the main component that occur due to spontaneous emission. The above methods i) and ii), therefore, have the following problems.
(1) Problems with the method involving superimposing the supervisory information signal on the main signal light.
i) With the superimposition of the supervisory information signal, the main signal light is in effect amplitude-modulated, which increases noise components and thus decreases the reception sensitivity of the receiving terminal.
ii) Amplitude modulation makes it difficult to stabilize optical repeater output.
(2) Problems with the method involving wavelength-division multiplexing of main signal light and supervisory information signal light for transmission.
i) A light-emitting device for emitting special wavelength light is needed at the transmitting side for the transmission of the supervisory information signal.
ii) When transmitting different main signal light beams by wavelength-division multiplexing, associated supervisory information signal light beams must also be multiplexed; this limits the number of main signal light beams which can be multiplexed together.